1. Field of the Invention
The present invention relates to a dielectric mirror of a reflecting type spatial light modulator used for a flat panel display, an optical arithmetic element and a video-projector and so on, and particularly relates to improvements of display quality of the spatial light modulator and mass-productivity of the dielectric mirror.
2. Description of the Related Art
As well known in the art, it is able to perform an incoherent-coherent conversion or a coherent-incoherent conversion of light by using a spatial light modulator, thus, there is proposed many applications to utilize the spatial light modulator such as a data parallel processing and a direct arithmetic processing of images. Further, the spatial light modulator is applicable to a display system such as a video projector by amplifying the intensity of image.
As a spatial light modulator like this, for instance, there is a type disclosed in Japanese Patent Laid Open 58-215626/1983 also in Appl. Phys. Lett., Vol. 22, No. 3, 1 February 1973, which is shown in FIG. 1 (A),
FIG. 1 (A) is a perspective view showing a construction of a reflecting type spatial light modulator 1 of the prior art, the general construction of which is employed in a spatial light modulator used in the present invention.
Referring to FIG. 1 (A), numerals 20, 24, designate glass substrates, and 18, 22, transparent electrodes interposed between the glass substrates 20, 24. At a side of the transparent electrode 18 from which a writing light impinges in a direction of an arrow F1, a photoconductive layer 16, a light-blocking layer 14 of an insulator, and a dielectric mirror 12 are laminated in the order. At another side of the transparent electrode 22 from which a reading light impinges in a direction of an arrow F2, a photomodulator layer 10 is formed. Further, an AC power source 26 is connected to the transparent electrodes 18, 22.
Next, the description is given to an operation of the above spatial light modulator 1.
In the operation, an AC voltage is supplied across the transparent electrodes 18, 22 from the AC power source 26. Under this condition, in general, if a light is irradiated to the photoconductive layer 16 in a direction shown by the arrow F1, the impedance of the photoconductive layer 16 is reduced in proportion to intensity distribution of the light or image, so that an AC voltage is applied to the photomodulator layer 10 according to thus reduced impedance distribution of the photoconductive layer 16.
Therefore, when a writing light carrying image information impinges on the photoconductive layer 16 in a direction shown as an arrow F1, a two-dimensional distribution of the AC voltage is applied to the photomodulator layer 10 corresponding to the two-dimensional distribution of intensity of the image information.
On the other hand, a reading light impinges on the photomodulator layer 10 in a direction shown as an arrow F2.
Accordingly, the reading light incident to the photomodulator is modulated in the photomodulator layer 10 to which the two-dimensional distribution of the electric field is applied corresponding to the two-dimensional distribution of intensity of image information carried by the writing light. The reading light reflected by the dielectric mirror 12 is again modulated in the photomodulator layer 10 when it passes therethrough, and exits out of the spatial light modulator 1 outputted toward outside in a direction shown as an arrow F3.
In the case of using a crystal structure or a liquid crystal having with a support body (for instance, a liquid crystal film) as the photo-modulator layer 10, a part of glass substrates or whole of it may be omitted. Further, the light-blocking layer 14 is optionally provided for preventing the reading light from leaking into the photoconductive layer 16 through the dielectric mirror 12 and from disturbing a two-dimensional distribution of the image charges formed in the photoconductive layer 16, otherwise such leakage would cause a reduction of the contrast ratio of the reproduced image.
As mentioned in the foregoing, a reflecting device for the reading light is indispensable to the reflecting type spatial light modulator 1. In principle, a conductive material such as a metal layer which blocks the electric field to be applied to the photoconductive layer 10 can not be employed as the reflecting device of the reading light. For this reason, the dielectric mirror 12 is employed as the reflecting device.
FIG. 1(B) is a sectional view showing a construction of a dielectric mirror of a prior art.
Generally, a dielectric mirror has a laminated construction of a first group of dielectric layers having lowrefractive indexes and a second group of dielectric layers having high refractive indexes laminated alternately, a thickness of each layer being determined to be .lambda./4 (.lambda.=wavelength) of the reading light. The thickness of each layer is expressed in an optical thickness thereof throughout the present specification.
Referring to FIG. 1(B), in this prior art, a dielectric mirror 12 is constructed by alternately laminating a number of low refractive index layers 30 of SiO.sub.2 as the first group and a number of high refractive index layers 32 of TiO.sub.2 as the second group, and further laminating a low refractive index layer 34 having .lambda./2 of the reading light on the top of the laminated construction.
In this dielectric mirror 12, the larger the numbers of laminated layers, the higher the reflectivity of the dielectric mirror 12, this means, the lower the transmittance of the dielectric mirror 12. However, if the wavelength of the reading light deviates from a designed wavelength of a spectral characteristic, the reflectivity of the dielectric mirror 12 will decrease, i.e., the transmittance thereof will increase. Therefore, it needs the light-blocking layer for blocking the reading light even when the intensity of the reading light is weak, except the case of using a monochromatic reading light. It is able to increase the reflectivity of the dielectric mirror 12 with respect to the reading light by substantially increasing the numbers of the laminated layers, however, it poses a problem of causing a reduction of productivity because of a thickness increase of more than a few .mu.m. In the case of using such a dielectric mirror with the reading light having a strong intensity for a video-projector, it will inevitably require a light-blocking layer of as thick as 1-10 .mu.m.
When the thickness of the light-blocking layer 14 and/or the dielectric mirror 12 becomes large, it requires a high driving voltage of the AC power source 26 thus causes a loss of the AC power source 26 from which the two-dimensional electric filed is applied to the photomodulator layer 10 corresponding to the change of conductivity of the photoconductive layer 16. This poses problems of degrading the contrast ratio and a resolution of the reproduced image, the latter is due to remoteness of the photomodulator layer 10 from the photoconductive layer 16 so that the electric field therefrom diverges at the photomodulator layer 10.
As a countermeasure to these problems, there disclosed a dielectric mirror in Japanese Patent Publication 3-217825/1991, wherein improvements of the contrast ratio and the resolution of the reproduced image are obtained by constructing the dielectric mirror from a first laminated portion having a light absorbing characteristic and a second laminated portion having a light reflective characteristic so as to have a function of the light-blocking layer.
Further, in Preprint for the 51th Meeting (1990) of the Japan Society of Applied Physics, page 751, the paper 26a-H3, or in U.S. Pat. No. 5,084,777, there is disclosed a dielectric mirror of Si--Ge alloy having a high refractive index and a large light absorbability.
However, the former poses a problem of obtaining a dielectric mirror having a high electric resistivity because of employing an Si layer therein, and the latter poses a problem of requiring a high cost for the materials to be used and for the production equipment as well as a problem of low productivity.